Digital radios often have many modes for communicating with one another. For example, a user may speak into a receiver of a first digital radio, where that radio receives the voice signal, processes it, and transmits the information from an antenna out over the air at RF frequencies. The over the air RF waveform is then received by a second radio at its antenna, converted to baseband, and processed to recover the voice information transmitted by the first radio. Conversely, two digital radios may be arranged in a mode whereby the first radio transmits information to the second (remote) radio over a two wire interface rather than as an over the air waveform. For example, a cable line may be used to link two radios in such a manner as to permit communication over the two wire interface rather than through the conventional RF antenna arrangement. This type of interface permits the user of one digital radio to transmit information directly to a second digital radio, which may then retransmit or rebroadcast that information to a number of radios as an over the air waveform. This type of communication interface is particularly useful in a number of military and commercial applications where the first radio operator may desire to broadcast information to a number of other operators, but because of interference, terrain, or covert activity, cannot transmit an over the air signal. Instead, the first operator will transmit the information bearing signal over the two wire interface to a second radio remotely located from the first radio, who will receive the information and retransmit at its antenna as an RF signal. Similarly, RF signal information may be received by the first radio at its antenna, sent to a second remote radio over the two wire interface, received by the second radio, and retransmitted as an over the air waveform at its antenna. The SINGCARS digital radio is an example of a type of radio which employs these various modes of communication. Radios employing these techniques are shown in commonly assigned, copending U.S. patent application Ser. No. 08/864,885, filed on May 1, 1997 by Pries, et al., entitled "Method and Apparatus for Voice Intranet Repeater and Range Extension", Ser. No. 08/857,990, filed on May 16, 1997 by Bertrand, et al., entitled "Radio Architecture for an Advanced Digital Radio in a Digital Communication System", and Ser. No. 08/850,231, filed on May 2, 1997 by Epstein, et al., entitled "Frequency Hopping Synchronization and Tracking in a Digital Communication System". These radios are often frequency hopping signal transmission systems, which are a type of spread spectrum system in which the wideband signal is generated by hopping from one frequency to another over a large number of frequency choices. The frequencies used are chosen by a code similar to those used in direct sequence systems. For general background on spread spectrum systems, reference is made to a text entitled Spread Spectrum Systems, 2nd edition, by Robert C. Dixon and published by Wiley-Interscience, New York (1984). In order to increase the efficiency of digital radios employing spread spectrum characteristics, digital engineers have raised the number of modulation levels and have generally dealt with spectral shaping, synchronization schemes and modulation/demodulation techniques.
A problem arises in the two wire communication interface as to how to effectively transmit and receive the information between the two digital radios. In order to communicate the information, the information bearing signal must be modulated for transmission over the interface and then demodulated to recover the information. Although there are many modulation systems, quadrature modulation is widely used to modulate both the amplitude and the phase of a carrier signal. In quadrature modulation, an in-phase (I) component and a quadrature phase (Q) component of a carrier signal are modulated and transmitted along with the information bearing signal in order to communicate within a particular system. Mapping circuitry, frequency mixers and band limiting filters shape and condition the resulting waveform in order to demodulate the transmitted signal to obtain the information bearing signal portion.
In the past, engineers have realized modulators/demodulators using analog circuit techniques. However, these circuits often suffered from signal deviation problems resulting from analog signal drift. In recent years, attempts have been made to construct modulator/demodulator circuits using digital circuit technology. In a digital radio, many different signal types, including control signals, packet data signals, and analog voice signals, are required to be modulated, transmitted across a communication interface, received by a receiver radio, and demodulated such that the information bearing portion of the transmitted/received signals can be understood. Furthermore, each of the various types of signals may require various modulation/demodulation methods according to the inherent characteristics of the particular signal to be transmitted. Consequently, it is desirable to obtain an improved two wire communications interface for determining the type of signal to be transmitted/received and the digital modulation/demodulation scheme to be performed on that particular transceived signal.